Coal combustion gas contains a small amount of mercury originating from coal. Mercury of this type may include low water-soluble metal mercury (Hg0), water-soluble divalent mercury (Hg2+(HgCl2)), and particulate mercury (HgP) attached to combustion ash. In an exhaust gas treatment system, most of the particulate mercury (HgP) can be removed by dust collecting equipment such as an electrostatic precipitator (EP) or a bag filter, and the water-soluble divalent mercury (Hg2+) can be removed by wet flue-gas desulfurization (FGD) equipment with high efficiency. However, the low water-soluble metal mercury (Hg0) is difficult to be removed with the EP and FGD equipment. Therefore, a method in which the metal mercury is changed to mercury chloride (that is, divalent mercury) in de-nitration equipment by adding hydrogen chloride to exhaust gas and then the mercury chloride is removed by absorption in the wet flue-gas desulfurization equipment is proposed.
For example, Patent Document 1 discloses a method in which a hydrogen chloride vaporizer directly heating an aqueous hydrogen chloride solution by hot air and generating hydrogen chloride is provided in an upstream side of denitrification equipment to supply the hydrogen chloride to exhaust gas in a flue, and metal mercury is changed to mercury chloride to be removed. In this technique, since the aqueous hydrogen chloride solution has a high oxidizing ability, the storage performance and the handleability are poor and thus there is a concern of corroding peripheral equipment. Accordingly, a technique of chlorinating the metal mercury by supplying ammonium chloride as a substitute thereof is proposed.
On the other hand, for example, Patent Document 2 discloses a method in which ammonium chloride powder is supplied to the bypass bypassing an economizer equipped with a boiler, and the ammonium chloride is sublimated by the heat of exhaust gas to obtain hydrogen chloride and ammonia, thereby supplying the hydrogen chloride to the exhaust gas.
However, in the method of directly supplying the ammonium chloride powder to the flue as in Patent Document 2, there is a problem that concentration of the hydrogen chloride in the exhaust gas deviates easily according to sublimation conditions of the ammonium chloride. Therefore, Patent Document 3 and Patent Document 4 disclose that liquid hydrogen chloride and ammonium are supplied to a flue through the supply ports. In addition, Patent Document 5 discloses that liquid ammonium chloride is supplied to a flue. The liquid hydrogen chloride, ammonia, and liquid ammonium chloride are evaporated by the heat of combustion gas to obtain hydrogen chloride gas and ammonia gas.